Investigating future changes in the volume budget of the Arctic sea ice in a coupled climate model

Arctic September sea ice cover has declined at a rate of 13% per decade since satellite observations began, and there is much interest in how this decline will continue in the future, both in terms of the predictability of ice cover in a given year, and in terms of the manner and timing of the transition to a seasonally ice-free Arctic. Global coupled models are arguably the best tool we have for making future projections of the Arctic sea ice, but generate a wide spread of projections of future ice decline. There are many factors potentially contributing to this spread, and it is becoming increasingly clear that as well as investigating ‘integrated’ quantities like ice cover and volume directly, it is also necessary to consider, compare and evaluate the underlying processes, and how they change.  

Here we consider the volume budget of the sea ice in the Arctic Basin in the HadGEM2-ES global coupled model (which was the UK/Met Office contribution to CMIP5), and how the budget components evolve during the 21st century under a range of different forcing scenarios. In terms of what happens per unit surface area of the ice, the processes that change most as the climate warms are summer melting at the top surface of the ice, and basal melting due to extra heat from the warming ocean. However, due to the declining ice cover these are not the budget components that contribute most to reductions in the ice volume, and the largest budget change is a reduction in the total amount of basal ice formation during the autumn and early winter.  

The choice of forcing scenario affects the rate of ice decline and the timing of change in the volume budget components, but for this model and for the range of scenarios considered for CMIP5, the mean changes in the volume budget depend on the evolving ice area, and are independent of the speed at which the ice cover declines.